[Bug c++/64758] [C++11] Give better error message when name of enum's base type cannot be resolved

[zeratul976 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64758

--- Comment #1 from Nathan Ridge <zeratul976 at hotmail dot com> ---
By comparison, clang's error is:

  test.cpp:1:14: error: unknown type name 'uint32_t'
  enum Waldo : uint32_t {
               ^